Many halogenated organic compounds and especially polychlorinated biphenyls are known toxins and are widespread environmental pollutants, as such compounds have been used in a variety of industrial and domestic applications. Such applications include electrical insulators, transformers, heat exchange fluids and dry cleaning solvents. PCB's in particular have been found to be a health hazard even at relatively low levels of concentration as such compounds tend to remain in the fatty tissues of a host once entry has been gained, eventually accumulating to toxic levels.
There are many conventional means to dispose of halogenated organic compounds and/or to dehalogenate halogenated organic compounds to less toxic materials. For example, PCBs have been disposed of by high temperature incineration. Such methods have proved unsatisfactory due, for example, to the extremely high temperatures involved to completely combust the higher chlorinated polychlorinated biphenyls and possibly resulting in the formation of even more toxic by-products such as dioxins.
There are a number of chemical processes for destroying PCBs. For example, U.S. Pat. No. 4,477,354 discloses a process which includes reaction of hydroxides of alkali and alkaline earth metals with PCBs and organic solvents with the end solvents being distilled off. Other chemical processes include the reaction of polychlorinated biphenyls with sodium naphthalimide generated in situ in ether-type solvents such as disclosed in U.S. Pat. No. 4,326,090; the reaction of polychlorinated biphenyls with alkali metal hydroxides in polyglycol or polyglycolmonoalkyl ethers such as disclosed in U.S. Pat. No. 4,400,522; the reaction of PCBs with nickel arylphosphine halide as disclosed in U.S. Pat. No. 4,400,566; the reaction of PCBs with alkalimercaptides as disclosed in U.S. Pat. No. 4,410,422; the reaction of PCBs with molten aluminum which is disclosed in U.S. Pat. No. 4,469,661; and, the reaction of PCBs with liquid sodium such as disclosed in U.S. Pat. No. 4,465,590. Despite the usefulness of such chemical processes in dehalogenating halogenated organic compounds, such processes require the use of hazardous materials and/or complicated reaction schemes also requiring separate isolation and separation steps prior to chemical reaction of PCBs.
An alternative approach to dehalogenation of polyhalogenated organic compounds by chemical methods is dehalogenation by electrochemical techniques. An electrochemical process for dehalogenation of alkyl halides in DMF is disclosed in Kaabak, et al. Org. Chem. U.S.S.R. 3:1 (1967). Other electrochemical processes include halogen removal by direct electron transfer from a cathode in a halogenated organic compound described in Feoktistov Chap. VII, Organic Electrochemistry, Balzen, et al. Eds. New York (1983); radical anion catalyst based dehalogenation described as a method for removing a halogen from an organic halogenated compound in Connors, et al, J. Electrochem Soc., 130:1120 (1983); and Fenn, et al. J. Electrochem. Soc., 123:1643 (1976) disclosing a process for oxidizing commercial mixtures of PCBs at high anodic potentials at a platinum electrode in a medium of aqueous acetonitrile and tetraethylammonium fluoroborate.
Such electrochemical dehalogenation methods described above have generally been regarded as hazardous, complex and expensive and thus commercially unattractive.
Other electrochemical processes include those described in U.S. Pat. Nos. 4,707,230 and 4,775,450 which involve the electrochemical based reaction of a compound capable of forming an iminium ion, e.g., N,N-dimethyl formamide, with a halogenated organic compound. The iminium ion forming compound and a source of halogenated organic compound are combined in a cell. The process also requires that an electroconductivity increasing solute soluble in the iminium ion forming compound be employed in the cell mixture which provides charged species upon dissolution as a means of establishing the desired electrical conductivity in the system, as the iminium ion forming compound does not by itself provide adequate electrical conductivity. Such solutes include tetra alkyl ammonium BF.sub.4, chlorides etc. A current at some predetermined peak voltage is then caused to pass through the cell to effect dehalogenation. The iminium ion forming compound is primarily employed as an electrolyte-solvent which dissolves charge-carrying species thereby providing a sufficiently electrically conductive medium to support the electrochemical dehalogenation reaction.
Such processes are based on controlled potential electrolysis and determinations of peak potential for the cathodic reduction of various halogenated organic compounds. These methods suffer from the requirement of relatively high concentrations of expensive electroconductive salts which are consumed in large quantities and are nonrecoverable, and which correspondingly produce reaction byproducts in large quantities which rapidly foul electrodes thereby inhibiting the reaction. These processes also consume large amounts of power due to the large amounts of salts employed. Such processes additionally require the electrochemical reaction to be closely controlled within a narrow potential voltage range by means of fragile and expensive reference electrodes to maintain a predetermined peak potential. Such processes also suffer from low electrochemical reaction rates and high equipment costs associated with their commercialization thereby leaving a continuing need for an efficient and economical process for dehalogenation of halogenated organic contaminants.
It is therefore an object of the present invention to provide a process for the dehalogenation of halogenated organic compounds which is devoid of hazards and uneconomical complexities associated with conventional prior art processes discussed above.
It is a further object of this invention to provide a process for the dehalogenation of halogenated organic contaminants in industrial and domestic applications.
Another object of the present invention is to provide an electrochemical process for the selective dehalogenation of organic contaminants.
An additional object of this invention is to provide such processes which selectively dehalogenate halogenated organic contaminants without affecting the physical and chemical characteristics of materials contaminated by halogenated organic compounds.
Additional objects and advantages of this invention will become readily apparent to those persons skilled in the art from the following discussion.
FIG. 1 illustrates a preferred embodiment of the present invention.
FIG. 2 illustrates an aspect of the invention.